1. Field of the Invention
The present invention relates to an apparatus which continuously applies an electron beam to a filamentary workpiece, and more particularly to an electron beam apparatus for curing a coating material that has been applied to optical fiber. The invention relates also to a method of fabricating optical fiber using such an apparatus.
xe2x80x9cFilamentary workpiece,xe2x80x9d as used herein, includes slender, continuous workpieces with a circular cross-sectional shape having a diameter of up to 5 mm, and preferably up to 1 mm; as well as slender, continuous workpieces with an elliptical or rectangular cross-sectional shape in which the long axis or horizontal dimension is up to 10 mm, and preferably up to 4 mm, and the short axis or vertical dimension is up to 3 mm, and preferably up to 1 mm.
2. Prior Art
As an electron beam passes through matter, it excites orbital electrons in the matter, triggering chemical reactions and generating secondary electrons and x-rays. Gradually, the electron beam loses energy to the matter and slows down. It also undergoes scattering, causing the radiation to advance in different directions and disperse. This tendency is especially striking in high-density matter such as solids. Electron beams endowed with such characteristics have been used in many diverse industrial applications. Applications in manufacturing industries in particular fall into two broad categories: those involving the use of chemical reactions triggered by electron beams, and those involving the use of heat generated with the loss of kinetic energy by the electrons. Examples of the former type of application include resin crosslinking reactions such as in tire rubber and polyethylene coatings on electrical wire, as well as resin curing reactions in coated paper and printing inks. Examples of the latter type of application include electron-beam melting in smelting operations and electron-beam welding in metalworking operations.
In the first type of application mentioned above, part of the energy of the irradiated electrons is consumed in the chemical reactions. Most of the remaining energy simply passes through, although a portion thereof becomes heat. Such heat is minimized to avoid the deteriorating effect it can have on the resin. By contrast, in the second type of application, substantially all of the kinetic energy of the electron beam is converted to heat when the electrons slow down within the irradiated object, and is thus available for heating the metal to its melting temperature. Hence, the role played by electron beams differs completely from one type of application to another.
At the same time, the systems used in each case for irradiating electron beams share a common feature; each has an electron generating means and an electron accelerating means of some type. However, the electron beam systems used in the first type of application described above are designed to irradiate a broad surface area so as to increase productivity, whereas the electron beam systems used in the second type of application are designed for spot irradiation to increase energy density. Therefore, in connection with the former type of application, relatively low acceleration voltage equipment makes use of a xe2x80x9ccurtainxe2x80x9d system having an electron generating means and an electron accelerating means that produce a broad electron beam band. Relatively high-acceleration voltage equipment makes use of a xe2x80x9cscanningxe2x80x9d system which has both an electron generating means and an electron accelerating means that together produce a narrow linear beam, and also an electron scanning means which distributes this beam over a broad area. In the latter type of application, the electron beam system typically includes an electron generating means and electron accelerating means that produce a linear beam, and also an electron focusing means that concentrates the beam toward a focal point.
In the first type of application, electron generation and acceleration are carried out in a vacuum, whereas irradiation is generally carried out at atmospheric pressure, which is more conducive to continuous treatment and thus advantageous in terms of productivity. In this type of system, a thin metal foil generally serves as the boundary separating atmospheric pressure from the vacuum, the electron beam being made to pass from the vacuum out to atmospheric pressure through the metal foil. The electron beam is strongly scattered as it passes through the metal foil, and thus diffuses following passage through the foil. However, this has not been a problem because the beam is intended to irradiate a broad area.
On the other hand, in the second type of application, because the electron beam must focus at one point, irradiation is generally carried out in a vacuum in which scattering does not occur. Most equipment of this type uses a batch treatment-type high-vacuum system in which the object to be irradiated is placed in an irradiation chamber and irradiation is carried out following evacuation of the chamber to a high vacuum. However, in a high-vacuum system, evacuation takes a long time, resulting in poor productivity. Hence, low-vacuum systems have been developed and adapted for practical use in which the irradiation chamber is connected to the high-vacuum electron-beam generator by a differential evacuating means, thereby making it possible to carry out irradiation even when the degree of vacuum in the irradiation chamber is low; i.e., even at a short evacuation time.
JP-B 5-50454 discloses art relating to the electron beam irradiation of a filamentary workpiece. While this prior-art reference does not specify the type of apparatus used, the energy of the electron beam or the degree of irradiation, it does describe the electron-beam curing of an optical fiber coating material. However, no technology has previously been arrived at for the continuous application of a focused electron beam to a filamentary workpiece under atmospheric pressure.
A variety of optical fibers are made, including quartz glass fibers, multicomponent glass fibers, and plastic fibers. Of these, large quantities of quartz glass optical fibers are used in a broad range of applications on account of favorable characteristics such as their light weight, low loss, high durability and large transmission capacity. However, the most common quartz glass optical fibers have a diameter of only 125 xcexcm and thus have a tendency to break with even the slightest scratch. Also, because transmission loss increases when the fiber is subjected to an external stress such as bending, a resin coating composed of a soft primary coating layer and a hard secondary coating layer surrounding the first layer is applied. Coating is typically carried out by using a die-coating process to apply a liquid resin over the bare optical fiber immediately after the fiber has been melt-drawn, then curing the resin by the application of heat or exposure to radiation (generally ultraviolet light). Secondary coating may be carried out by a coating and curing process conducted either subsequent to or concurrent with coating and curing of the primary coating. The coated optical fiber is also commonly colored with an ink for the sake of identification. A number of coated optical fibers, typically four or eight, are gathered into a bundle, which is coated with a liquid resin. The resin is then cured by the application of heat or by exposure to radiation such as ultraviolet light, thereby giving an optical fiber tape.
Coating materials proposed for such use include urethane acrylate-based ultraviolet-curable resin compositions. JP-B 1-19694, JP No. 2522663 and JP No. 2547021 disclose liquid compositions of UV-curable resins composed of a urethane acrylate oligomer, a reactive diluent and a photopolymerization initiator.
Optical fibers are being drawn today at higher speeds to enhance the productivity of the manufacturing process. This rise in speed has been accompanied by an increase in the energy per unit time required to cure the resin coating. But because improvements in the output of UV lamps commonly used for curing the resin have been unable to keep pace with such progress, it has been necessary to install multiple UV irradiation units in series. Unfortunately, the rate of production achievable is often effectively capped by the amount of space available for such installation.
Electron beam curing is generally regarded as more energy efficient than UV curing. Yet, this is true only in those cases where the irradiated object is broad and the electron beam, even if it diffuses, strikes some portion of the irradiated object, such as in the resin curing of coated paper or printing ink. When a prior-art curtain-type electron beam system is used to irradiate a filamentary workpiece with an electron beam, even if the direction of the electron beam band is aligned with the direction of the filamentary workpiece, the large degree of scattering by the electron beam as it passes through the metal foil results in a very small proportion of electrons striking the filamentary workpiece, and thus a low energy efficiency. When a scanning-type electron beam system is used, even if the beam is not scanned and is instead held stationary on the filamentary workpiece, considerable scattering of the electron beam as it passes through the metal foil likewise occurs, again resulting in a low efficiency. This problem has been overcome in the electron-beam crosslinking of polyethylene-coated electrical wire by having the wire double back repeatedly as it travels so as pass through the irradiation system a number of times. However, this approach is useless in optical fibers because, when an optical fiber that has been coated with a liquid composition is electron beam cured, such bending back and forth of the fiber before curing is complete damages the coating.
Electron beam scattering can be avoided by irradiation in a vacuum, but the vacuum-type irradiation systems used in welding are batch-type systems. Such systems are incapable of carrying out continuous treatment, and so cannot be used for curing optical fiber coating. Moreover, existing electron beam welding systems are unable to uniformly irradiate the periphery of a filamentary workpiece during travel.
Another problem with the electron beam curing of optical fiber coating is that the electron beams alter germanium dopant in the optical fiber core, increasing transmission loss.
It is therefore an object of the invention to provide an apparatus which is capable of uniformly, efficiently, and continuously applying electron beam irradiation to a traveling filamentary workpiece, and particularly an electron beam system for treating a filamentary workpiece which can accommodate higher fiber drawing speeds and does not compromise the transmission characteristics of the optical fiber. An additional object of the invention is to provide a method of fabricating optical fibers using such an apparatus.
Accordingly, in a first aspect, the invention provides an electron beam system for treating a filamentary workpiece, which system includes an electron beam irradiation chamber having a plurality of openings through which passes a filamentary workpiece, an electron beam generator having an electron generating means, an electron accelerating means and an electron focusing means, a communicating section which connects the electron beam generator with the electron beam irradiation chamber, and differential evacuating means of at least one stage for holding the pressure within the electron beam generator below that within the electron beam irradiation chamber. Electrons are generated by the electron generating means, accelerated by the electron accelerating means, and focused by the electron focusing means into a beam, which passes from the electron beam generator through the communicating section to the electron beam irradiation chamber where the beam is directed at a filamentary workpiece that passes continuously through the openings in the chamber.
The electron beam system preferably has a plurality of electron beam generators arranged at approximately equal intervals around the electron beam irradiation chamber.
It is also preferable for the electron beam system to have a purge gas feeding means which is situated on the communicating section at a position adjacent to the electron beam irradiation chamber and feeds purge gas so as to make the pressure within the communicating section in the vicinity of the purge gas feeding higher than the pressure within the electron beam irradiation chamber.
According to one embodiment of the electron beam system according to the invention, the pressure within the electron beam irradiation chamber is typically about atmospheric pressure.
According to another embodiment of the inventive system, the pressure within the electron beam irradiation chamber is kept below atmospheric pressure by another differential evacuating means mounted at an opening through which the filamentary workpiece passes.
In the above embodiment of the invention in which the interior of the electron beam irradiation chamber is typically at about atmospheric pressure, the electron beam system preferably has a means for placing the electron beam irradiation chamber under an inert gas atmosphere, and preferably a helium atmosphere. In a system in which the irradiation chamber is under an inert gas atmosphere, the distance from the inlet where the electron beam enters the irradiation chamber to the filamentary workpiece is preferably less than about 60 mm, and most preferably less than about 20 mm.
In the electron beam system according to the first aspect of the invention, it is preferable to dispose a means for reflecting or absorbing the electron beam in the electron beam irradiation chamber such as to face across the filamentary workpiece and toward the electron beam generator.
In the electron beam system of the invention, the filamentary workpiece is typically an optical fiber coated with a liquid composition of an electron beam-curable resin. Moreover, the accelerated electrons preferably have a maximum energy of up to about 120 keV and an average energy of at least about 60 keV.
In a second aspect, the invention provides a method of fabricating optical fiber, which method includes the step of applying a liquid composition of an electron beam-curable resin coating material to an optical fiber, followed by the step of using the electron beam system of the first aspect of the invention to irradiate the applied composition with an electron beam in which the accelerated electrons have a maximum energy of up to about 120 keV and an average energy of at least about 60 keV so as to cure the coating material.
According to one embodiment of the method of the invention, the electron beam is applied within a nitrogen atmosphere and the distance from the inlet where the electron beam enters the irradiation chamber to the optical fiber is less than about 20 mm.
According to another embodiment of the inventive method, the electron beam is applied within a helium atmosphere and the distance from the inlet where the electron beam enters the irradiation chamber to the optical fiber is less than about 60 mm.
In the optical fiber fabricating method of the invention, the electron beam-curable resin is typically composed primarily of urethane acrylate, and the electron beam is preferably irradiated in an atmosphere having an oxygen concentration of less than about 1,000 ppm and such as to give an absorbed dose of about 10 to about 100 kGy.
According to another embodiment of the electron beam system of the invention, the filamentary workpiece is a bundle of optical fibers that is coated with a liquid composition of an electron beam-curable resin. The accelerated electrons in this embodiment preferably have a maximum energy of up to about 160 keV and an average energy of at least about 120 keV.
In a third aspect, the invention provides a method of fabricating optical fiber tape, which method includes the step of applying a liquid composition of an electron beam-curable resin coating material to a bundle of optical fibers, followed by the step of using the electron beam system of the first aspect of the invention to irradiate the applied composition with an electron beam in which the accelerated electrons have a maximum energy of up to about 160 keV and an average energy of at least about 120 keV so as to cure the coating material.
According to one embodiment of the above optical fiber tape fabricating method, the electron beam is applied within a nitrogen atmosphere and the distance from the inlet where the electron beam enters the irradiation chamber to the bundle of optical fibers is less than about 20 mm.
In another embodiment, the electron beam is applied within a helium atmosphere and the distance from the inlet where the electron beam enters the irradiation chamber to the bundle of optical fibers is less than about 60 mm.
In the optical fiber tape fabricating method of the invention, the electron beam-curable resin is typically composed primarily of urethane acrylate, and the electron beam is preferably irradiated in an atmosphere having an oxygen concentration of less than about 1,000 ppm and such as to give an absorbed dose of about 10 to about 100 kGy.